#include<stdio.h>

int main()
{
    int l,n,m;
    scanf("%d %d %d",&l,&n,&m);
    int f[500002]={0};
    f[n+1]=l;
    int sum=0;
    for(int i=1;i<=n;i++)
    {
        scanf("%d",&f[i]);
    }
    int left=1;
    int right=l;
    while(left<=right)
    {
        int mid=left+(right-left)/2;
        int count=0;
        int last=0;
        for(int i=1;i<=n;i++)
        {
            if(f[i]-f[last]<mid)  count++;
            else  last=i;
        }

        if(count<=m)
        {
          sum=mid;
          left=mid+1;
        }
        else right=mid-1;
    }
   printf("%d",sum);
}